1. Field of the Invention
This invention relates to pile driving apparatus.
2. Description of the Prior Art
The specification of our U.S. Pat. No. 3,828,866 describes a pile driving apparatus wherein the impulse driving apparatus or hammer is enclosed in a casing, the bottom of which is provided with a downwardly extending tubular guide portion or pile sleeve the lower end of which is provided with an outwardly flaring frusto-conical bell portion. The pile sleeve serves to guide the apparatus on to and to receive the top of the pile. The provision of a pile sleeve, so long as it has sufficient length and its internal dimensions approximately correspond to the external diameter of the pile to be driven, enables the pile driving apparatus to be operated to drive the pile while being supported only by the top of the pile fitting in the pile sleeve with its top resting against the pile cap or anvil which is struck by the hammer blows.
The pile driving apparatus described in the aforesaid specification is specifically for driving piles under subaqueous conditions and has a casing which is closed at its upper end and is provided with means for introducing compressed air into the casing to exclude water therefrom when the casing is submerged. However, similar apparatus of which the casing is not necessarily closed at its upper end is also suitable for use in free air, the apparatus again being supported by the top of the pile to be driven without any separate guide for the pile driving apparatus being necessary.
Pile driving apparatus as described has been made in a range of sizes, some of the larger units for driving tubular steel piles of 7 feet diameter being about 45 feet high over the casing and pile sleeve and weighing about 200 tons. It will be appreciated that such massive pile driving apparatus, or even much smaller sizes, are subject to and deliver heavy blows while being guided on the top of a pile. Thus the pile sleeve must be very strong and is generally reinforced by vertical ribs spaced around and projecting radially inwardly from the internal surface of the sleeve, the innermost edges of the ribs lying on a circle of a diameter approximately equal to the external diameter of the pile. To reduce the risk of the top edge of the tubular steel pile being distorted by impact with the innermost edges of the ribs, cut simply from steel plate, during positioning of the pile sleeve on the pile, the said innermost edges may be widened by securing other plates transversely thereto to provide the ribs with a substantially T-shaped cross-section; the transverse portion of the T may be flat or curved to approximate to the curvature of the pile.
It is seldom that all the piles to be driven are of the same diameter and, as for reasons of cost and other practical considerations it is not feasible, especially when pile driving from a vessel or rig on the ocean, to provide a series of pile driving units equipped with pile sleeves suitable for the different piles or even to provide interchangeable pile sleeves for a common hammer unit, it is the usual practice to provide the unit with a pile sleeve corresponding to the maximum diameter pile to be driven, and to reduce the effective sleeve diameter, as and when required for smaller diameter piles by welding inwardly projecting extension ribs to the innermost edges of the ribs and welding a guide ring to the frusto-conical bell portion of the pile sleeve to provide a lead in to the smaller diameter of the circle on which the innermost edges of the extension ribs lie. Such welding operations are expensive; they have to be carried out in closed-in spaces under hot and difficult conditions by expert welders in order to ensure that the wells can withstand the operating forces without reduction of safety standards. The welded on guide ring is secured so that the effective length of the pile sleeve is not reduced, reduces the lead-in angle of the bell portion and consequently increases the difficulty of guiding the apparatus on to the top of a pile.